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Trend 2.5
Trend 2.5 of the Beryllium Group (Group IIA) Trends is that BeX2 (X = F, Cl, Br, or I) are covalent polymers, which are readily hydrolyzed and are Lewis acids forming adducts BeX2L2 (L = Lewis base). Magnesium, calcium, strontium, and barium halides are essentially ionic and are soluble in water. Beryllium Beryllium exhibits extremely different characteristics compared to the other group II element halides. Be bonding is dominantly covalent compared to Mg, Ca, Sr, and Ba which are ionic. This then means that BeX2 (X = F, Cl, Br, or I) is a covalent polymer while MgX2, CaX2, SrX2, and BaX2 are ionic salts. The Uniqueness Principle and the Periodic Law explain this behavior in addition to looking at electronic orbitals to understand its lewis acidic properties. Periodic Law Be also exhibits differences in periodic trends such as atomic radius, ionization energy, electron affinity and electronegativity. Properties of the Group 2 (IIA) Elements:Miessler, Gary, and Donald Tarr. Inorganic Chemistry. 4thth ed. Upper Saddle River, NJ: Prentice Hall, 2011. Uniqueness Principle Be, as a second row element is extremely small in size. This means that Be can get very close to a F-, Cl-, Br-, or I- anion, close enough to polarize the anion’s electron cloud and allow better overlap and greater sharing of the electron wave function, characteristic of a covalent bond. Beryllium’s small size leads to its inability to have a high charge to size ratio and thus cannot have complete charge separation to form Be2+.Rodgers, Glen. Descriptive Inorganic, Coordination, and Solid State Chemistry. 2nd ed. N.p.: Brooks Cole, 2002 Bonding OrbitalsDavyson, Sam. "S Block." High Storrs Chemistry Department. http://hs.davyson.com/media/52/Notes%20-%20S%20block.pdf. Based on bonding orbitals, the periodic law, and the uniqueness principle, it makes sense that BeCl2 is a linear covalent polymer. The figure at right represents the bonding that occurs in BeCl2 as a linear polymer.Clark, Jim. "Some Beryllium Chemistry Untypical of Group II." ChemGuide. http://www.chemguide.co.uk/inorganic/group2/beryllium.html. It is important to note that Be is covalently bonded to two chlorine atoms and coordinated to two others. The bond angles of the Cl—Be – Cl unit are 98.2˚ which suggest that the bonds are enlongated in the direction of the chain axis.Cotton, Albert, Geoffery Wilkinson, and PAul Gaus. Basic Inorganic Chemistry. 3rdrd ed. New Delhi: John Wiley & Sons, 2004 Thus Be is acting as a lewis acceptor and Cl is acting as a lewis donor with formula BeX2L2 (L = X in this case acting as a lewis base). As shown from the bonding orbital figure above, Be is electron deficient and has room to accommodate 2 more lone pairs. Because Be acts as a lewis acid, it can act as an acceptor to lewis donors including the Cl- anion, as well as others such as H2O. This forms an adduct of BeCl2(H2O)2. BeCl2(s) + H2O(l) à BeCl2(H2O)2 In contrast, based on periodic trends, thus their lower electronegativity and larger size Mg, Ca, Sr, and Ba are ionic and form ionic bonds with the halides. These atoms have a higher charge to size ratio and can have complete charge separation. These compounds form lattice structures as is typical of ionic compounds. When these ionic halides are placed in water they dissociate into their ions and are highly soluble in a polar solvent like water. CaCl2 (s) + H2O(l) à Ca2+ (aq) + 2 Cl- (aq) The structures of a few MCl2 structures are given below for comparison. BeCl2 MgCl2 CaCl2 References Category: Group IIA Trends